Method to accelerate drying of wood



United States Patent 3,507,053 METHOD TO ACCELERATE DRYING OF WOOD Marcel Goulet, 1529 Garnier St., Quebec 6, Quebec, Canada No Drawing. Filed May 23, 1968, Ser. No. 731,626 Int. Cl. F26b 7/00 U.S. Cl. 3413.8 18 Claims ABSTRACT OF THE DISCLOSURE A method for accelerating drying of wood in which the wood is subjected to a perpendicular-to-grain compression at a pressure in the zone of the elastic limit of the wood and for a short period of time. Then the pressure is released and the wood dried at a temperature which is higher than the temperature at which the wood would normally be damaged when not subjected to such compression treatement.

This invention concerns a perpendicular-to-grain compression treatment of wood in order to facilitate the passage of fluids through it. In practice this process will be highly useful to accelerate the drying of Wet or green lumber. The method will be described 'with reference to the kiln drying of yellow birch, as this industrial operation is of prime commercial importance in North America. However, it is evident that the same compression treatment could also be applied successfully to woods of other species.

One of the main limits to the acceleration of industrial drying of wet lumber rests in its tendency to be degraded by internal or surface checking as well as by collapse, a defect which is manifested by a higher than normal shrinkage and a wavy appearance of the boards. These phenomena increase markedly with the intensity of drying, that is with the temperature and the moisture gradients used. They vary a great deal from one species, and even from one lot of wood, to another. So it is recognized that simple air seasoning can produce collapse in certain eucalypts. Other species behave so only during normal kiln-drying, but one must then be contented with a relatively slow drying, beginning at low temperatures between 110 and 140 F.; this is the case, for example, with redwood, black walnut, and different oaks and mahoganies. Finally, another group of species, including North American yellow birch and European beech, dry rather well artificially; but it has still been impossible to dry them directly at high temperatures, that is at temperaures near or above the boiling point of water (210 to 250 F.); before using this technique their lumber has to be dried, at temperatures of 150 F. or less, down to a moisture content corresponding to the fiber saturation point of wood (about 30% M.C.).

There are excellent reasons for seeking either to accelerate the drying process or to reduce the degradation of the lumber produced or both. Indeed the cost of this operation, taking into account the huge quantities of raw material involved, are such that any improvement in this area must result in considerable direct benefits for the forest industry. Furthermoe, the indiect advantages may still be far from negligible. For example, a shortening of the drying time will not only reduce the direct costs of this operation but will further allow a reduction of the lumber stocks with all the connected financial, maintenance and insurance charges and/ or increase the flexibility of the wood products industry through shorter delivery times of dry stock on a highly competitive maket.

The transverse compression treatment which is the subject of the present invention and which shall now be described, has the characteristic of being quite simple and of requiring only a'small capital investment while allowice ing to reach the sought effect, that is: facilitate the passage of fluids through wood and/or reduce the substantial losses due to wood degradation by a too severe drying schedule.

Essentially the method consists to subject wood to a sufficiently high transverse or perpendicular-to-grain pressure, that is to say approaching or exceeding the elastic limit of the wood in the particular conditions where the pressure is applied. The transverse crushing strength of wood, hereby defined as the stress or strain level above which the final product would be permanently damaged, makes up the only limit to the pressure that can be used.

Any method for applying the required transverse pressure can be used, its effect being virtually instantaneous. So, for example, the wood can be compressed quickly or slowly; cold or warm; before or during the period of drying; only once, at given steps, or by cycling; on all faces of the wood pieces, or in only two of their sides; on the full length of the boards like in a platen press, or only in limited zones at one time like between rollers. However, it is evident that it will be more often practical to have the boards passing quickly one after the other between two or four parallel pressure rollers, these being part of a conveying system.

The present invention will now be illustrated by means of the following example.

EXAMPLE Kiln-drying of yellow birch lumber High grade, green, flat-sawn yellow birch boards 1 and 2 inches thick, 8 inches wide, and 7 feet long were divided into matched groups. The first group was subjected to transverse compression treatment followed by kiln-drying to 8 percent moisture content. The second group of similar thicknesses was kiln-dried in the same kiln charge directly from the green condition and served as controls.

The equipment for the compression treatment was designed to pass lumber at any chosen speed between 10 and 50 feet per minute between two 5-inch rollers placed parallel, one over the other. This equipment is capable of exerting sufficient transverse pressure to produce a thickness reduction of 12.5 percent in an 8-inch wide board through a double acting 6-inch hydraulic cylinder acting on the top roller. The amount of compressive strain, and the change in board thickness and width immediately after compression, were measured with a strain gage transducer system.

The dryer employed has been constructed specially for research purposes on high temperature kiln-drying of lumber. The drying schedule used is summarized in As to air speed, it was maintained at 350 feet: 10 feet per minute; this value may appear low considering the current tendency toward an increase in air circulation in commercial dryers. Nevertheless, it was justifiable here since the stack depth was 8 inches only.

Prior to kiln-drying, a transverse pressure producing a strain of 7.5 and 5.2 percent was applied to l-inch and 2-inch material respectively. These pressures are in the zone of the elastic limit of the boards treated (Table 2).

TABLE 2.DIMENSIONAL BEHAVIOR OF YELLOW BIRCH LUMBER Z-inch Control (percent) l-inch Control Treated (percent) (percent) Treated (percent) thickness +0. 12 0. 35 P ercent shrinkage from green to 8 percent M Thickness Width Average drying time green to 8 percent M.C. (hrs.)

However, as soon as the compressed portion of the board passed through the rollers, most of the deformation recovered. After the initial two hours of steaming (which is part of the high temperature schedule), the remaining deformation in l-inch boards was only 0.35 percent, whereas the thickness of the control samples increased by 0.12 percent; the remaining deformation in 2-inch boards completely recovered and board thickness increased slightly.

The drying time for treated and non-treated material was essentially the same. As expected, the control material was degraded to such an extent that it became non-usable. On the contrary, the treated material compared favorably with lumber that is kiln-dried at conventional temperatures, although the drying time required was only onesixth. Total shrinkage of prepressed lumber from green to 8 percent M.C. was less in both thickness and width than of the controls (Table 2). Thus, this kiln-drying experiment clearly proved the effectiveness of a perpendicularto-grain compression treatment for facilitating the kilndrying of yellow birch.

What I claim is:

1. A method to facilitate the passage of fluids through wood, comprising the steps of compressing said wood transversely of the wood grain for a short period of time, allowing the wood to recover from said compression and placing said wood in an environment in which process r variables selected from pressure and temperature differ from those within the wood, so as to effect transfer of fluid between said wood and environment.

2. A method as claimed in claim 1, wherein the compressing step is effected at a pressure in the zone of the elastic limit of the wood in the particular conditions in which the pressure is applied.

3. A method as claimed in claim 1, wherein the compression to which the wood is subjected, is slightly less than the pressure which would permanently damage the Wood.

4. A method to accelerate drying of wood normally subjected to damage when dried at high temperature, comprising the steps of subjecting wood to a perpendicular-tograin compression for a short period of time, allowing the wood to recover from said compressionand dryingsaid wood at a temperature which is higher than the temperature at which the wood would normally be damaged.

5. A method as claimed in claim 4, wherein the dry bulb drying temperatureis in the range of 210 F. to 250 F. i

6 A method as claimed in claim 4, wherein the drying temperature is about 215 F.

7. A method as claimed in claim 5, wherein the Wood is subjected to steaming at substantially atmospheric pressure, then dried in air to about 7% moisture content and subsequently conditioned to about 8% moisture content.

8. A method as claimed in claim 4, wherein the compressing step is elfected at a pressure in the Zone of the elastic limit of the wood in the particular conditions in which the pressure is applied. 7

9. A method as claimed in claim 4, wherein the compression to which the wood is subjected, is slightly less than the pressure which would permanently damage the wood.

10. A method as claimed in claim 5, wherein the compressing step is effected at a pressure in the zone of the elastic limit of the wood in the particular conditions in which the pressure is applied.

11. A method as claimed in claim 5, wherein the compression to which the wood is subjected, is slightly less than the pressure which would permanently damage the wood.

12. A method as claimed in claim 7, wherein the compressing step is elfected at a pressure in the zone of the elastic limit 'of the wood in the particular conditions in which the pressure is applied.

13. A method as claimed in claim 7, wherein the compression to which the wood is subjected, is slightly less than the pressure which would permanently damage the wood.

14. A method as claimed in claim 4, wherein the wood in the shapeof lumber and the compressing step is effected by passing said lumber between at least one pair of pressure rollers. g

15. A method as claimed in claim 14, wherein the lumber is passed between said rollers at a speed of between 10 and feet per minute.

16. A method as claimed in claim 14, wherein said pressure rollers effect a compression on said lumber of between 5 to 8 percent its thickness.

17. A method as claimed in claim 15, wherein said pressure rollers effect a compression on said lumber of between 5 to 8 percent its thickness.

18. A'method as claimed in claim 4, wherein said wood is yellow birch.

References Cited UNITED STATES PATENTS 128,387 6/1872 Gyles 34- -l4 XR 1,677,963 7/1928 =Ford 34143 XR 2,711,591 6/1955 Wellmar 34-18 3,183,606 5/ 1965 Gustafsson et al 3470 FOREIGN PATENTS 624,023 5/ 1949 Great Britain. 684,915 4/ 1964 Canada.

FREDERICK L. MATIESON, 111., Primary Examiner R. A. DUA, Assistant Examiner 

